In medical care, the correct placement of a medical device such as a catheter or a guide wire in a patient has become increasingly important for a number of reasons. In the case of an infusion catheter, for one example, medications may need to be targeted to or for specific organs or areas of the body. In some instances, it may be important to place a catheter sufficiently near the heart where a particular blood flow rate ensures adequate dilution and mixing of infused fluids. Alternatively, a catheter or other internally-positioned medical device may simply need to be disposed in the right place to function; as for example, an enteral feeding tube within the stomach. Use of a medical device position location and/or guidance system may thus provide for skilled and less skilled practitioners to more simply and/or accurately and reliably position a medical device such as a catheter and in some instances without the use of an x-ray or additional ancillary procedures to confirm the location of the catheter or device. Additionally, the use of a medical device position location system additionally may provide for maintenance of the sterile field, a critical aspect in placing catheters or other internally positioned medical devices.
Accordingly, a variety of systems have been developed to attempt to indicate location or position of catheters or other medical devices disposed within the body of a patient. Relatively reliable location devices have made use of x-ray or fluoroscopy; however, these devices may expose the patient and/or caregiver to undesirable amounts of radiation. As a consequence, a variety of different systems have been attempted to more continuously and accurately indicate location of a catheter or other medical device with a goal of reducing and/or replacing the use of x-rays and as an alternative to fluoroscopy. However, such systems thus far developed still suffer from various drawbacks.
Electromagnetic catheter position location devices have been the subject of research and development. Some such locating systems have used an AC driven coil in the catheter tip with external sensor coils. A disadvantage of such a conventional catheter tip driven system has been the need for heavy or thick wires running into the catheter to carry necessary drive current to generate a sufficient electromagnetic signal for external sensors. This has precluded the use of such a system with smaller diameter catheters or other such smaller medical devices. Other position location systems have used a fixed magnet (or DC) on a catheter tip with external sensor coils. A significant disadvantage to such a fixed magnet location system has been that the magnet would necessarily be very small, and as such would generate a very small signal from the tip of the catheter. Additionally, DC magnet systems put an additional charge into the patient and as a consequence, other magnetic fields in the vicinity may create significant interference problems for such a system. Furthermore, the field of such a magnet drops off extremely quickly over distance and thus cannot be sensed more than a few inches deep into the patient's tissue. This results in some concern about the depth of the placement in the subject.
Some locating systems have made use of AC driven external coils and a sensor coil in the catheter tip. One such AC drive system has been described including driving two coils simultaneously; however, those respective coils were specified as having been driven at two different frequencies so that the coil drives are not additive and the sensor demodulates the two different frequencies as two independent values. Yet another AC drive system has been described driving two coils simultaneously in quadrature which simulates a single spinning coil; however, this system may only indicate the orientation of the sensor in the x-y plane and its relative position in that plane.
This statement of background is for information purposes only and is not intended to be a complete or exhaustive explication of all potentially relevant background art.